Process for the production of 18F-2-deoxy-2-fluoro-D-glucose

ABSTRACT

Process for the production of 2-deoxy-2-fluoro-D-glucose and the corresponding  18  F-compound by the reaction of acetyl hypofluorite or the corresponding  18  F-compound with 3,4,6-tri-O-acetyl-D-glucal followed by hydrolysis. Process includes the production of the hypofluorite compound at ambient temperature.

BACKGROUND OF THE INVENTION

The United States Government has rights to this invention pursuant toContract Number DE-AC02-76CH00016 between the United States Departmentof Energy and Associated Universities, Inc.

This invention relates to 2-deoxy-2-[¹⁸ F]fluoro-D-glucose (¹⁸ FDG) andto methods of preparing it. More particularly, it relates to novelprocedures for the preparation of this known compound.

The development of a rapid synthetic procedure to ¹⁸ FDG using ¹⁸F-labeled elemental flourine ([¹⁸ F]F₂) coupled with the development ofpositron emission transaxial tomography (PETT) and appropriatemathematical models has made it possible to measure local cerebralglucose metabolism in man non-invasively. This has generated intenseinterest in the biomedical community in the use of ¹⁸ FDG and PETT tostudy the correlation of metabolism and function in a variety of humanpathologies as well as normal activity.

As a result of this interest, many cyclotron (accelerator)-PETT centershave instituted, or are in the process of instituting, the targetry andsynthesis system required for producing this radiotracer. These ¹⁸ FDGsynthesis systems have been based on minor modifications of the ¹⁸ FDGsynthesis originally reported by Ido et al., J. Org. Chem. 42: 2341(1977) and J. Label. Cmpds. Radiopharm, 14: 175 (1978). A major problemin meeting increasing demands for this tracer is that manycyclotron-PETT centers have medical cyclotrons which do not have theoptimal deuteron energies for ¹⁸ F production via the ²⁰ Ne(d,α)¹⁸ Freaction described by Casella et al., J. Nucl. Med. 21: 750 (1980).This, together with the low chemical yield from the original ¹⁸ FDGsynthesis (about 10%) imposes a limitation on the capabilities of manycenters to synthesize sufficient ¹⁸ FDG for their own needs and hasnecessitated the supply of ¹⁸ FDG from centers where it can be producedto institutions using the tracer product which are located within a 2 to3 hour shipping radius. It is apparent then, that the development of animproved synthesis of ¹⁸ FDG would make it possible for institutionswith small medical cyclotrons or other accelerators to producesufficient quantities of ¹⁸ FDG for their own daily use and would allowthe production of multiple dose batches of ¹⁸ FDG by institutions withcyclotrons of higher production capacity.

THE INVENTION

A procedure has now been discovered for the preparation of ¹⁸ FDG whichpermits the rapid, facile production of this compound in highly purifiedform at a yield which is generally about double the yield which can beachieved with the conventional method.

In accordance with the invention, ¹⁸ FDG is produced by the reaction of3,4,6-tri-O-acetyl-D-glucal (TAG) with ¹⁸ F-labeled acetyl hypofluorite,CH₃ COO¹⁸ F, followed by acid hydrolysis. The latter compound isproduced from [¹⁸ F]F₂ by a novel method.

The production of acetyl hypofluorite was first described by Rozen etal., J.C.S. Chem. Comm. 443 (1981). The procedure employed was to reactnitrogen diluted fluorine with sodium acetate and acetic acid intrichloromonofluoromethane at -78° C. It has now been discovered thatacetyl hypofluorite can be prepared at ambient temperature, e.g. 20° to40° without the use of a halogenated hydrocarbon solvent. In theprocess, fluorine in a reaction inert gas such as nitrogen or neon ispassed into a mixture of acetic acid and a molar excess of alkali metal,alkaline earth metal or ammonium salt of acetic acid at ambienttemperature and the mixture held at the selected temperature for 20 to40 minutes to produce the desired product. The product is normally notisolated but is utilized in situ.

The yield of product may be determined by adding an aliquot of thesolution to excess KI solution and titrating the liberated iodine withsodium thiosulfate. The chemical yield is normally about 80%. Theradiochemical yield is about one half of that value since each moleculeof fluorine produces one molecule of fluoride salt in addition to theacetyl hypofluorite.

The presently preferred reagent is ammonium acetate which isconveniently generated by the addition of ammonium hydroxide to aceticacid.

The following table shows typical yields of acetyl hypofluorite withvarious acetate salts in acetic acid utilizing a mixture of fluorine andneon.

                  TABLE I                                                         ______________________________________                                        Cation      Reaction time                                                                            Yield of CH.sub.3 COOF                                 ______________________________________                                        Na.sup.+    13         52%                                                    Na.sup.+    23         64%                                                    Na.sup.+    46         67%                                                    NH.sub.4.sup.+                                                                            23         79%                                                    K.sup.+     23         77%                                                    Cs.sup.+    23         71%                                                    None        23         44%                                                    ______________________________________                                    

The ¹⁸ F-acetyl hypofluorite is converted to ¹⁸ FDG by reaction withTAG, preferably a slight molar excess to insure complete reaction,followed by hydrolysis of the acetyl groups. The preferred procedure isto add TAG to the solution containing the ¹⁸ F-acetyl hypofluorite. Thereaction is spontaneous, starts immediately and goes rapidly tocompletion.

The intermediate 2-deoxy-2-[¹⁸F]-fluoro-1,3,4,6-tetra-O-acetyl-α-D-glucopyranose may be isolated, butis normally hydrolyzed to ¹⁸ FDG with dilute aqueous acid, for example,1 to 2.5N hydrochloric acid at 100° C. to 135° C. for 10 to 20 minutes.Other acids can be employed, preferably dilute inorganic acids such assulfuric acid.

The product can be isolated by any convenient procedure such aschromatography as illustrated in the examples. The yield by thepresently preferred methods is about 20%, a factor of 2 higher than theprevious synthesis.

This invention has been described with reference to acetyl substitutedcompounds since these are most readily available, inexpensive andconvenient to use. Those skilled in the art will recognize that thereactions are general ones and that compounds substituted with otheracyl groups can be employed.

EXAMPLE I

Synthesis of CH₃ COO¹⁸ F from [¹⁸ F]F₂

[¹⁸ F]F₂ (30 to 40 μmol) prepared by the procedure of Casella et al.,cited above, was purged from the target through a glass reaction vesselcontaining a solution of ammonium hydroxide (58%, 0.010 ml) in aceticacid (15 ml) over a period of 25 minutes to give a solution of CH₃ CO₂¹⁸ F. The vessel (0.43 in I.D.×9 in. high) was fitted with a Teflon frit(0.43 in. diameter) through which 12 small holes (0.0145 in.) weredrilled. This design provided efficient gas dispersal.

The yield of CH₃ CO₂ ¹⁸ F produced by this method was determined bytransferring the acetic acid solution into excess of 1 M KI solution andtitrating the liberated I₂ with 0.01 N Na₂ S₂ O₃. The chemical yield was80% (24-32 μmol) and the radiochemical yield was 40%. For the synthesisof ¹⁸ FDG, the acetic acid solution containing CH₃ CO₂ ¹⁸ F was usedimmediately after purging of the target contents was completed.

EXAMPLE II

Synthesis of 2-deoxy-2-[¹⁸ F]fluoro-D-glucose From ¹⁸ F-labeled AcetylHypofluorite

To a solution of the hypofluorite (24-32 μmol) prepared as describedabove was added 25 mg of TAG in 1 ml of acetic acid. This solution wasevaporated to dryness, 3 ml of 2 N HCl was added and the mixture heatedat 120° for 12 min. Activated charcoal (10 mg) was added, the acid wasevaporated, 3 ml of aqueous acetonitrile (0.3% H₂ O) added and themixture transferred to a column (0.75×10 cm) of silica gel (Merck No.9385) followed by a 2 ml rinse with the same solvent, a forecut (6.5 ml)taken and discarded and the product eluted with about 15 ml of solvent.The solvent was evaporated, 1 ml of H₂ O (USP) added and this was alsoevaporated. Saline was added and the solution passed through a milliporefilter (0.22 μm). Thin layer chromatography (CH₃ CN:H₂ O, 95:5) showedthe product to have a radiochemical purity of 98%, the impuritiesprobably being partially hydrolyzed, 2-deoxy-2-[¹⁸F]fluoro-1,3,4,6-tetra-O-acetyl-α-D-glucopyranose. The methylsilylderivative and radiogas chromatography of the product as described inSweeley et al., J. Am. Chem. Soc. 85, 2497-2507 (1963) showed theradioactivity to be congruent with the mass peaks corresponding to thesilylated α- and β- anomers of 2-¹⁸ FDG. HPLC (Bio-Rad HPLC carbohydrateanalysis column, 85° C. H₂ O, flow 0.6 ml/min) also confirmed theidentity of the product as 2-¹⁸ FDG with retention time of 8 min.

The absence of 2-deoxy-2-[¹⁸ F]fluoro-D-mannose (2-¹⁸ FDM) was verifiedby synthesizing this compound independently from2-deoxy-2-fluoro-3,4,6-tri-O-acetyl-β-D-mannopyranosyl fluoride by themethod of Ido et al., cited above, forming its trimethylsilyl derivativeand analyzing by glc (10% SE-30, 6 ft.×1/8 in.; flow 20 ml/min; 180°).The retention times of the silylated derivatives of 2-FDG were 24 min(α) and 30 min (β), while the retention times of 2-FDM derivatives were25 min (α) and 34 min (β).

From 24-32 μmol of acetyl hypofluorite used in this synthesis, 12-16μmol (2.2-2.9 mg, 50% chemical yield) of 2-FDG is produced. Theradiochemical yield is 20% based on total ¹⁸ F recovered from thetarget. Thus from 350 mCi of ¹⁸ F, 45 mCi of 2-¹⁸ FDG is obtained at theend of a 70 minutes synthesis (EOS). This corresponds to a specificactivity of 15.5-20.5 mCi/mg at EOS.

What is claimed is:
 1. A process for the production of ¹⁸F-2-deoxy-2-fluoro-D-glucose in yields of about 20% based on total ¹⁸ Fwhich comprises reacting ¹⁸ F-acetyl hypofluorite with3,4,6-tri-O-acetyl-D-glucal and thereafter hydrolyzing the resultingtetraacetyl compound in dilute aqueous acid to remove the acetyl groups.2. A process for the production of ¹⁸ F-2-deoxy-2-fluoro-D-glucose whichcomprises the steps of:1. reacting F₂ in a reaction inert gas withacetic acid and a molar excess of an alkali metal, alkaline earth metalor ammonium salt of acetic acid at a temperature of from 20° to 40° C.for from 20 to 40 minutes,
 2. adding 3,4,6-tri-O-acetyl-D-glucal andallowing the mixture to react, and
 3. hydrolyzing the resultingtetraacetyl compound in dilute aqueous acid to remove the acetyl groups.3. A process as in claim 2 wherein the acetic acid salt is ammoniumacetate.